Hello Arch Enemy,
I have done tons of research short of calling up the company directly and can't find a definitive answer for you.
I am thinking you should be able to use the 6-pin PCI-E connector in the 4-pin port on the video card and it should be keyed the same way and have no problem. However, I can't say 100% for sure. I believe the 8-pin connector is specifically for powering the CPU on newer motherboards.
If you can use the 6-pin connector, you will have to check the video card to make sure there isn't any obstruction at the end of the card that will get in the way of the extra pins.
Your best bet to get a definite answer would be to contact the power supply company directly either by e-mail or phone. They should be able to tell you specifically what you will be able to do. There may be some kind of 6-pin to 4-pin converter you can get for it.
Hope that helps!
